1. Field of the Invention
The present invention relates to a polymer nanocomposite membrane and a fuel cell using the same, and more particularly, to a polymer nanocomposite membrane having a reduced permeability of water or methanol and a fuel cell using the same having improved energy density and fuel efficiency.
2. Discussion of the Related Art
Direct methanol fuel cells (DMFC) using a methanol solution as liquid fuel may be a future source of clean energy to replace fossil energy. Also, a DMFC may operate at room temperature, and it can be miniaturized and sealed. Thus, the DMFC may have multiple uses, including zero-emission vehicles, home power generating systems, mobile communication equipment, medical appliances, military equipment, aerospace industrial equipment, and portable electrical devices.
The DMFC produces direct current by an electrochemical reaction of methanol and oxygen. FIG. 1A shows a typical DMFC structure.
Referring to FIG. 1A, a hydrogen ionic conductive membrane 11 is interposed between an anode and a cathode.
The thickness of the hydrogen ionic conductive membrane 11 may be 50-200 μm, and it may be mainly made of a solid polymer electrolyte. Catalyst layers 12 and 13 are formed on an anode supporting layer 14 and a cathode supporting layer 15, respectively. The supporting layers 14 and 15 are made of carbon cloth or paper, and their surfaces are treated so that water to be transferred to the hydrogen ionic conductive membrane 11 and water generated by the reaction, can easily pass therethrough while supplying a reaction gas or liquid. Reference numeral 16 denotes a bipolar plate, which acts as a current collector, having grooves for injecting gas.
When a reaction fuel is supplied to the DMFC, an oxidation reaction occurs in the anode, converting methanol and water into carbon dioxides, hydrogen ions, and electrons. During this process, the hydrogen ions are transferred to the cathode via the hydrogen ionic conductive membrane 11.
On the other hand, a reduction reaction occurs in the cathode, wherein oxygen molecules in the air receive the electrons to be converted into oxygen ions. Then, the oxygen ions react with the hydrogen ions from the anode to be converted into water molecules.
In the above DMFC, the hydrogen ionic conductive membrane 11 may be a solid polymer membrane, and it separates fuels to be supplied to the anode and the cathode, while transferring the hydrogen ions produced in the anode to the cathode.
Nafion is typically used as the solid polymer membrane. The solid polymer membrane may contain water because it may be formed of a polymer that has a hydrophobic backbone and hydrophilic group-containing side chains. The water may form clusters that the hydrogen ions migrate through. Therefore, solid polymer membranes with increased water content may be preferred for effective hydrogen ion transfer.
In a DMFC fueled by an aqueous methanol solution, the solid polymer membrane may swell, depending on the fuels' methanol concentration. With this swelling, non-oxidized fuel may migrate from the anode to the cathode through the solid polymer membrane, thereby lowering the fuel cell's performance and wasting fuel.
In order to solve this problem, a solid polymer membrane may be developed for DMFCs.
U.S. Pat. Nos. 5,795,496, 6,194,474 and 6,510,047 disclose a method of lowering aqueous methanol solution permeability in a solid polymer membrane by using rigid and heat-resistant polymers. This method may significantly lower the methanol solution's permeability. However, since this method may considerably reduce the polymer membrane's ionic conductivity, the fuel cell's performance, such as power density, may be considerably lowered.
U.S. Pat. No. 5,919,583 discloses a method for forming a solid polymer membrane with dispersed inorganic particles. According to this method, the added inorganic materials may lower the membrane's hydrogen ionic conductivity.